Heretofore, removal of fine particles from transparent thermoplastic resin has relied on, for example, use of microfiltered reaction materials including monomers, catalysts, solvents, etc., or microfiltration of a prepared resin solution or a solution obtained by redissolving the produced resin in a solvent.
These methods require use of a very precise filter set to remove fine particles by filtration. When the resin solution is to be filtered, the pressure drop in filtration becomes higher as its viscosity becomes higher. This leads to the defect that the amount of the resin solution to be treated decreases, and the filtration efficiency is poor. Filters are naturally blocked up by particles, and in order to perform efficient purification by filtration, the filters must be frequently exchanged, and moreover, the amount of the resin solution to be filtered is very large. A method improved in both operation and cost has therefore been sought heretofore.
In the purification of a transparent thermoplastic resin solution containing a substantially water-immiscible solvent, a centrifugal separating method is used in the prior art to separate the resin solution from water used for washing. Usually, the purpose of this centrifugal separation is to separate from the resin solution phase the washing water which is used to remove the unreacted monomers, catalyst and other water-soluble impurities in the resin solution. In this operation, an intermediate layer exists between the resin solution phase and the washing water phase (=aqueous phase) which are centrifuged.
In this case, the manner is employed such that the intermediate layer between the resin solution phase and the aqueous phase is handled as the resin solution phase side, and water washing and centrifugation are repeated.
The reasons for this are as follows. Since the water-soluble impurities are usually dissolved in the resin solution in amounts corresponding to the distribution ratio, the operation is essential to lower the concentrations thereof successively, thereby decreasing the water-soluble impurities. Further, the thickness of the intermediate layer increases with increasing the residual amount of the water-soluble impurities, and in the event that those impurities are withdrawn as an aqueous phase, the yield greatly lowers (although varying depending on the residual amount of the water-soluble impurities, in the case of a general step of removing unreacted monomers, catalyst and other water-soluble impurities by washing with water, if all of intermediate layer is handled as an aqueous phase side, 10% or more of yield reduction occurs), and also load of waste water treatment increases remarkedly.
The atmosphere of the conventional centrifugal operation does not essentially require an environment where a specific particle control has been made. Air generally contains at least 1 million/cft of particles, and at least 100,000/cft of particles are present in the atmosphere of conventional production process. The resin solution is charged electrostatically during moving it through pipes and the like, and even if a slight amount of air contacts with the resin solution, the resin is contaminated with a large number of particles. However, the resin is conventionally processed under such environmental atmosphere and used, and there is no problem on transparency, safety, etc. of the resin in conventional use manner or purpose of use.
As is apparent from various literature references, conventional centrifugation is a step of efficiently remove water-soluble impurities through an operation of separating an aqueous solution from a resin solution phase by centrifugation, and is merely considered a method for practicing an operation of standing separation or the like with good producibility.
Therefore, conventional centrifugation is not recognized as an operation which removes particle as in the process of the present invention. Further, whether or not the operation is effective to remove particle can be first determined by testing a centrifugal separation operation under atmosphere where specific particle control has been made and investigating in detail the particle number in each of phases centrifuged.
Thus, in the conventional centrifugation, in order to achieve removal of water-soluble impurities and high yield of resin simultaneously, the intermediate layer is handled as a resin solution side, and the conventional centrifugation is not substantially an effective method as a removal method of particle. Moreover, the centrifugal force, the separation treatment time, etc. for removal of free water are naturally insufficient for the purpose of particle removal.